Fuel cells have recently been drawing attention as power sources that restrain global warming caused by exhaust gas, and some types of fuel cells have been put to practical use. A fuel cell generates electric energy by reacting hydrogen with oxygen. Instead of directly using oxygen as a cathode, a typical fuel cell system supplies air to a cathode to use oxygen in the air. Some of the water and nitrogen generated at the cathode move from the cathode to the anode by passing through an electrolyte membrane. That is, reverse diffusion occurs. Thus, if the fuel cell continues operating, the concentration of water and nitrogen at the anode increases. When the concentration of water and nitrogen at the anode surpasses a certain level, the power generation efficiency of the fuel cell is reduced. To prevent or restrain this, an anode purge is typically executed for discharging water and nitrogen that has accumulated in the anode. If anode off-gas is directly released to the atmosphere when performing the anode purge, the hydrogen concentration in the exhaust gas is too high. Patent Document 1 and Patent Document 2 each disclose an exhaust gas processing device that dilutes anode off-gas with cathode off-gas before discharging, so as to lower the hydrogen gas concentration in the exhaust gas.
FIG. 15 illustrates the exhaust gas processing device disclosed in Patent Document 1. The exhaust gas processing device of FIG. 15 includes a dilution container 51, an anode off-gas inlet passage 52, a dilution gas passage 57, a dilution gas release hole 58, a gas mixture discharge hole 59, a partition plate 53, and a communication gas passage 56. The axis of the dilution container 51 extends substantially horizontally. The anode off-gas inlet passage 52 has an anode off-gas release hole 52a for releasing anode off-gas into the dilution container 51. The dilution gas passage 57, through which dilution gas flows, extends through the dilution container 51 along the bottom. The dilution gas release hole 58 releases dilution gas that has flowed through the dilution gas passage 57 into the dilution container 51. The gas mixture discharge hole 59 causes the dilution gas passage 57 to discharge gas mixture, which is formed by mixing anode off-gas and dilution gas in the dilution container 51. The partition plate 53 is arranged to be substantially vertical in the dilution container 51, so as to divide the interior of the dilution container 51 into an upstream chamber 54 and a downstream chamber 55. The communication gas passage 56 connects the upstream chamber 54 to the downstream chamber 55. The anode off-gas release hole 52a is formed to release anode off-gas toward the partition plate 53.
FIG. 16 illustrates the exhaust gas processing device disclosed in Patent Document 2. The exhaust gas processing device of FIG. 16 includes a hydrogen inlet port 60, a retention container 61, a hydrogen discharge port 62, a dry air inlet portion 63, a first blocking body 64, a second blocking body 65, a coupling arm 66, an anode off-gas pipe 67, an urging weight 68, and a dilution gas pipe 69. The retention container 61 has a retention chamber 61a. The hydrogen inlet port 60 of the anode off-gas pipe 67 introduces anode off-gas containing hydrogen discharged from the anode of a fuel cell into the retention chamber 61a, so that the gas stays in the retention chamber 61a. Hydrogen retained in the retention chamber 61a is discharged to the dilution gas pipe 69 through the hydrogen discharge port 62, and diluted with cathode off-gas, serving as dilution gas. The hydrogen is then discharged to the outside. Unless anode off-gas is introduced through the hydrogen inlet port 60 into the retention chamber 61a, dry air is introduced into the retention chamber 61a through the dry air inlet portion 63.
The first blocking body 64 limits the introduction of hydrogen through the hydrogen inlet port 60 into the retention chamber 61a. The second blocking body 65 restricts discharge of hydrogen through the retention chamber 61a through the hydrogen discharge port 62. When hydrogen is introduced through the hydrogen inlet port 60 to the retention chamber 61a, the coupling arm 66 causes the first blocking body 64 to operate together with the second blocking body 65, so that hydrogen in the retention chamber 61a is not discharged through the hydrogen discharge port 62. A center portion of the coupling arm 66 is pivotably supported by the dilution gas pipe 69 with a shaft member 66c and a base 66d. The coupling arm 66 has a slightly widened L-shape when viewed from the side. That is, the coupling arm 66 includes a first arm 66a on the upstream side of the flow of hydrogen and a second arm 66b on the downstream side.
The angle between the first arm 66a and the second arm 66b is set to such an angle that the second blocking body 65 opens the hydrogen discharge port 62 when the first blocking body 64 closes the hydrogen inlet port 60, and that the second blocking body 65 closes the hydrogen discharge port 62 when the first blocking body 64 opens the hydrogen inlet port 60. The urging weight 68 is fixed to an upstream end of the first arm 66a. The own weight of the urging weight 68 urges the first blocking body 64 to close the hydrogen inlet port 60 via the first arm 66a. The mass (weight) of the urging weight 68 is set such that, during hydrogen purging, hydrogen that has reached the upstream surface of the first blocking body 64 through the anode off-gas pipe 67 pushes the first blocking body 64 toward the retention chamber 61a, so as to allow the hydrogen inlet port 60 to be opened.
When anode off-gas is introduced into the retention chamber 61a of the exhaust gas processing device of FIG. 16, the second blocking body 65 closes the hydrogen discharge port 62. When the second blocking body 65 is arranged at the open position to discharge anode off-gas in the retention chamber 61a to the hydrogen discharge port 62, the first blocking body 64 closes the hydrogen inlet port 60. Therefore, high concentration of hydrogen is prevented from being discharged during the anode purge. However, the exhaust gas processing device of FIG. 16 has a complicated structure. Further, the maintenance for allowing the first blocking body 64 and the second blocking body 65 to move smoothly, or rotate smoothly, is troublesome.
Unlike the exhaust gas processing device of FIG. 16, the exhaust gas processing device of FIG. 15 requires no structure for preventing anode off-gas from being introduced into the upstream chamber 54, and no structure for preventing gas from being released through the dilution gas release hole 58. However, in the structure shown in FIG. 15, the dilution gas release hole 58, which introduces dilution gas into the upstream chamber 54, and the gas mixture discharge hole 59, which discharges diluted anode off-gas from the downstream chamber 55, are holes formed in the single dilution gas passage 57. Therefore, it is difficult to properly adjust the amount of dilution gas introduced into the upstream chamber 54 and the amount of gas discharged from the downstream chamber 55. Anode off-gas and dilution gas are introduced into the upstream chamber 54, which is defined by the partition plate 53. Thus, anode off-gas during anode purge can be moved along the flow of dilution gas introduced into the upstream chamber 54. That is, a high proportion of the anode off-gas is insufficiently diffused in the upstream chamber 54, moved to the downstream chamber 55 with dilution gas, and discharged through the gas mixture discharge hole 59. In other words, the hydrogen concentration can be temporarily raised during the anode purge.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-31998    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-344470